It was initially feared to have been deliberately stolen by an international crime syndicate which understood its worth, and the Foundation reportedly offered a substantial reward for its safe return.

The 12ft-high sculpture was later discovered to have been melted down and sold for as little as £1,500.

Police are now appealing for information to help them retrieve the Sundial sculpture, estimated to be worth up to half a million pounds.

Richard Calvocoressi, director of the Henry Moore Foundation, said: “We are deeply saddened about the loss of Sundial from the gardens of Henry Moore's former home and are helping the police in the hope it may be recovered.

“We take our care of Henry Moore's sculptures extremely seriously and have installed heightened security measures here in recent years.

“We would be grateful if anyone with information would contact the police.”

The sculpture had been placed in the 72-acre grounds of The Henry Moore Foundation, the former home of the sculpture which is now open to the public.

It was positioned to be enjoyed from the sitting room of the property, and was overseen by the Foundation, an art charity Moore set up before his death in 1986.

Detective Inspector Paul Watts, who is leading the investigation said: "This 'sundial' sculpture is a valuable piece and we are very keen to speak to anyone who may have seen it since 4.30 pm yesterday (July 10th).

A spokeswoman for the Henry Moore Foundation said they would not be commenting on the theft, amid fears speculation could impede the investigation or their collection could be targeted.

Alice Farren-Bradley from the Art Loss Register, said more people were now registering sculptures kept outside as they became increasingly aware and concerned about metal theft.

She said of the Sundial: “It is of such importance to the estate and the general public as a whole, as a piece by a celebrated artist which sadly cannot be recreated.

“It would be worth so much more as a piece of art than its base metal value.”

She added “all hope was not lost”, as another Moore sculpture, "Three Piece Reclining Figure", was finally found last year after being reported missing in 2001.

Last year, a bronze entitled Two Forms (Divided Circle) by Dame Barbara Hepworth, one of Moore's contemporaries, was stolen from Dulwich Park. Police feared it too was targeted by metal thieves.

In February this year, a nude bronze sculpture by Moore sold for £19.1m at auction, believed to be a record price for the artist.